I had this issue with Waves plugins not showing up on a new install of Studio One 4 Pro. Under Effects the Waves folder was not even showing. Other plugins from EastWest and others were there but no Waves at all. After reinstalling all my Waves stuff which did nothing (turns out the issues wasn't Waves), I uninstalled Studio One completely, and even manually when into my user folder and renamed the AppData settings (didn't want to trash them YET) in order to have a fully clean uninstall, and when I reinstalled Studio One, all was there.. Might be something to try.
